Deactivation by coke formation is a serious problem in many industrial processes in which organic molecules are converted over catalysts based on microporous materials such as zeolites, aluminophosphates (AlPO) or silica-aluminophosphates (SAPO). Examples of such processes are cracking of hydrocarbons, alkylation of hydrocarbons and conversion of oxygenates to hydrocarbons. The existing techniques for reducing this kind of deactivation all affect the catalytic properties of the microporous materials.
To improve properties, such as product yield, selectivity, activity or stability, the microporous materials are often modified by catalytically active materials, promoters or stabilisers when applied as catalysts. Examples are the addition of Ni, W, Pd to a Y-zeolite in a hydrocracking catalyst, or the addition of Ni to a SAPO in the conversion of methanol to ethylene and propylene.
Well known oxygenate conversion processes that make use of catalysts based on microporous materials are Methanol-to-Olefins (MTO) and Methanol-to-Gasoline (MTG). In these processes, methanol is converted to hydrocarbon molecules. In the MTO process, described in U.S. Pat. No. 4,499,327, the desired products are olefins such as ethylene, propylene and butylenes. If the process is aimed at the production of propylene, it is sometimes called Methanol-to-Propylene (MTP). The catalysts commonly used in these processes are based on H-ZSM-5 zeolites or SAPO-34. Such a process is described in U.S. Pat. No. 6,518,475. In the MTG process, described in U.S. Pat. No. 3,894,104, methanol is converted to gasoline. A H-ZSM-5 based catalyst is preferred here.
A process related to the methanol conversions described above is TIGAS. This process, which is further described in U.S. Pat. Nos. 4,481,305 and 4,520,216, integrates the synthesis of methanol and DME from synthesis gas with the conversion of methanol and DME to gasoline, thus obtaining a process in which fuel is synthesised directly from synthesis gas.
The economy of the TIGAS, MTG, MTO and MTP processes depends critically on the stability of the zeolite based catalyst that produces the fuel product. Suppression of catalyst deactivation is therefore of crucial importance for these processes.
The deactivation rate depends on the nature of the reactants and the products, the process conditions and the catalyst formulation. A catalyst deactivated by coke formation can often be regenerated by heating it in an oxygen containing gas stream (typically air or diluted air), which burns off the coke, thus restoring the activity of the microporous catalyst. Very often, the deactivation is so fast that the microporous catalysts have to be continuously regenerated in the chemical plant during operation. In these cases, fluidised bed reactors are often preferred, since in these reactors it is a relatively simple process withdrawing a part of the catalyst for regeneration and thereafter reintroducing it in the reactor.
If deactivation is slower, two or more parallel fixed bed reactors can be used. In such a process, one of the reactors is regenerated while the other(s) is/are used for production. An alternative is to temporarily shut down the plant and regenerate the catalysts. In both cases, it has to be decided how often catalyst regeneration must be performed. This interval is the cycle length and it is the duration a catalyst can be operated without a significant loss in feed conversion. In the MTP process described by Rothaemel, M. et al. (ERTC Petrochemical Conference, March 2003, Paris), incorporated herein by reference, this cycle length is approximately 700 hours. Water may be added to the feed to improve process performance.
Regeneration usually leads to a deterioration of catalyst performance because the regeneration frequently is incomplete and may damage the catalyst. Furthermore, deactivation is inevitably connected with carbon loss in the process, which implies a lower product yield. Finally, the need for catalyst regeneration is an important cost factor, as it makes the process less efficient and always requires additional installations requiring considerable investments. For these reasons, it is desirable to suppress catalyst deactivation by coking as much as possible.
A commonly applied strategy to suppress coke formation is changing the process conditions. For example, a hydrocracking catalyst has a much longer lifetime (more than one year) than a FCC catalyst (approximately one minute), due to the presence of hydrogen in the hydrocracking process. In U.S. Pat. No. 4,520,216, it is disclosed that catalyst deactivation in the TIGAS process is reduced by adjusting process parameters.
Another strategy to avoid coke formation is a change of the properties of the catalyst. For example, catalysts based on dealuminated aluminosilicates normally have a longer lifetime in hydrocarbon conversion processes. However, the dealumination significantly affects the catalysts characteristics (activity/selectivity), since both the number and average activity of the active acid sites are reduced. The use of catalysts with a different micropore structure is sometimes a possibility to avoid coke formation, but this approach implies a change in catalytic characteristics as well, and can have major consequences for the process design.
The connection between the catalytic characteristics and deactivation properties is a problem in catalyst and process design, and always results in a compromise between deactivation and catalytic properties.
Therefore, there is a strong need for a process for oxygenate conversion, in which deactivation by coke formation of catalysts based on microporous materials is suppressed without changing other catalytic properties.